1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile.
2. Description of the Related Art
In an image forming apparatus in which toner is frictionally charged to develop a latent image on a photoreceptor, a charging amount of the charged toner is not always constant, the toner is naturally neutralized when an image forming apparatus body is not operated and is left, and the charging amount of the toner is gradually decreased. For this reason, after the image forming apparatus body is turned off, when the image forming apparatus body is operated after time elapses to some extent, the charging amount of the toner is decreasing, and thus an image density increases.
In a conventional image forming apparatus, at the time of power-on, a predetermined patch image is formed on an intermediate transfer member or a photoreceptor, and an image density thereof is read using an optical reflection detection sensor or the like. When the image density deviates by the extent equal to or more than a predetermined degree, tone correction is performed. Specifically, a patch having a plurality of density levels is formed, and a tone density is corrected based on a density of a plurality of patch images.
As the image density, there are density correction based on a digital patch and density correction based on an analog patch (see Japanese Patent Laid-Open No. 2001-117297). Here, the digital patch means a patch image formed by performing exposure based on an exposure unit and development based on a development unit after an image bearing member is charged. The analog patch means a patch image formed by changing a difference between a development voltage of the development based on the development unit and a charging potential of the image bearing member without performing the exposure based on the exposure unit.
A latent image potential of the photoreceptor, particularly, a bright potential is changed according to a temperature and a durable state. For this reason, in detection of the change of the image density based on the digital patch, when a potential sensor capable of measuring the potential of the photoreceptor is mounted, it is possible to form the patch image at a constantly accurate development contrast, and thus it is possible to accurately detect the density change. However, when the potential sensor is not mounted, the patch image is formed at a slightly different image contrast each time. As a result, although an actual digital patch density is temporarily changed but the density is not changed essentially, it may be detected that the density is changed.
In the detection of the change of the image density based on the analog patch, since the density change is detected in a state where there is no bright potential, it is possible to detect the change of the actual density purely based on the change of the charging amount of the toner without receiving an influence of the change of the bright potential. However, generally, when the analog patch is output, image density unevenness easily occurs due to mechanical change since the analog patch is output at a contrast smaller than a contrast of the digital image. When the image is actually output, the image is formed on a predetermined screen in a digital manner but not an analog manner. For this reason, the change of the image density may be different from the change of the image density actually output onto a sheet.
As described above, even when the change of the image density is performed at any of the analog manner and the digital manner, it may be erroneously detected that the image density is changed even when the change of the density does not actually occur, and image density correction adjustment (tone correction) may be performed. When the image density correction adjustment (the tone correction) is performed on all density tones and all screen patterns, a lot of time and toner are consumed. For this reason, in such detection of the change of the density, it is preferentially required to detect that the density is actually changed.